Install Mid-Circuit Interrupters in PV Array

The Tesla Mid-Circuit Interrupter (MCI) is used within PV strings and arrays to meet the Rapid Shutdown requirements of NEC Article 690.12. For information on how the Tesla MCI functions with Powerwall 3 as a Photovoltaic Rapid Shutdown System (PVRSS), see Appendix D: Solar Rapid Shutdown.

Note

The Tesla MCI is also referred to as the Tesla Solar Shutdown Device.

Note

There are two models of Tesla MCI, MCI-1 and MCI-2. Both perform the same function, but they have unique specifications (see Specifications) and installation methods (outlined in the following installation instructions).

Figure 1. MCI-1 (left) and MCI-2 (right)

MCI Locations

The quantity and placement of MCIs necessary to meet Rapid Shutdown requirements in NEC Article 690.12 (B) and Canadian Electrical Code (CE Code), C22.1, is governed by the UL 3741 (PV Hazard Control System or PVHCS) listing applicable to various array types. Refer to the UL 3741 Application Addendum for a complete list of systems in the US and Canada that have been certified to UL 3741 when installed together with Powerwall 3 and Tesla MCI. Always refer to the applicable PVHCS datasheets and references included in the addendum for detailed instructions to ensure adequate compliance and safe operation.

Warning

It is critical that the minimum number of required MCIs is installed; without the appropriate number of MCIs, the MCIs may not function as intended which could lead to equipment damage.

Warning

Where it is allowable to install only (1) MCI per string, MCI-1 must be used.

Installation Best Practices

Ensure that Tesla MCIs are not stored prior to installation in environments with moisture or dirt
Install the Tesla MCI in line with the PV modules using standard Staubli Type MC4 certified compatible connectors
Ensure the MCI is minimum 12.7 mm (1/2 in) from glass or backside of a PV module
For flat plate solar panel installations, ensure the MCIs do not directly contact the roof. For Solar Roof installations, see the Solar Roof Installation Manual for proper mounting guidance
For easier servicing of flat plate solar panel installations, install the MCIs on the perimeter of the array if possible
Always secure the MCI with appropriate wire management
Ensure no more than 5 MCIs are connected per PV string
Ensure that total DC circuit length, from + MPPT terminal to - MPPT terminal (including module wire leads, jumper wires and all wiring within the array boundary for any individual string), does not exceed 160 m for single strings or paralleled strings
- Total DC circuit length refers to the entire round trip wire length, from inverter to the roof, then back to the inverter
- Ensure paralleled strings are the same length, or as close in length as possible (if the paralleled strings are different lengths, it is likely the MCIs will not function properly; this likelihood increases the greater the difference in paralleled string lengths)
Keep a completed string diagram indicating the layout of the array and the placement of each MCI (either a paper copy, electronic copy available in the field, or both)

Installing MCIs

Connect the positive and negative leads of the MCI to the corresponding positive / negative MC4 terminals of the PV modules or solar roof modules.
For flat plate solar panel installations, attach the MCI under the module frame.
1. To attach the MCI using ZEP Home Run wire clips, install the (2X) clips to the module frame and clip both ends of the MCI leads, then slot the MCI underneath the metal frame.
  Figure 2. MCI-1 Attached with Wire Clips
  Figure 3. MCI-2 Attached with Wire Clips
  Note
  Install the wire clips at least two inches from the end of the MC4 connector, but no more than six inches from the connector. Ensure the MCI is not directly contacting the roof.
  CAUTION
  Do not twist the MC4 connectors on MCI-2. Twisting the connectors damages the electrical contacts inside, likely resulting in MCI failure.
2. To bolt the MCI to an existing hole in the frame, align the MCI through hole with the frame and use the appropriate mounting hardware as defined in MCI Mounting Hardware Sizing .
  Note
  This option is only available for MCI-1.
  Follow all module manufacturer’s instructions before mounting the MCI to the module frame. Ensure there is at least 0.5 in (13 mm) between the MCI and the module substrate, and only mount the MCI using existing holes in the module frame.
  For rail PV systems, use any of the mounting holes to mount the MCI directly to the rail (see MCI Mounting Hardware Sizing for mounting hardware sizes) or use the manufacturer’s module level power electronic mounting bracket and mount the MCI to the bracket.
To install MCI-1 on Solar Roof installations, mount the MCIs directly to the roof, directly above the Tile Footlaps and positioned at a slight angle as pictured below. Use the appropriate mounting hardware as defined in the table below.
Figure 4. MCI-1 Installed with Solar Roof Tiles (V3 and earlier generations)
Figure 5. MCI-1 Installed on End of Solar Roof V3R String
Table 1. MCI Mounting Hardware Sizing
Through Hole Hardware
A Wood Screw (No. 8)
Screw (up to M4 (5/32"))
B Bolt (up to M8 (5/16"))
C Nail (8D)
Screw (up to M4 (5/32"))
To install MCI-2 on Solar Roof installations, attach each lead of the MCI to the corresponding Staubli MC4 connector, and place an MC4 cap on each of the remaining MC4 connectors. Secure the cables in the wire management clips.
CAUTION
Do not twist the MC4 connectors on MCI-2. Twisting the connectors damages the electrical contacts inside, likely resulting in MCI failure.

Through Hole	Hardware
A	Wood Screw (No. 8)
A	Screw (up to M4 (5/32"))
B	Bolt (up to M8 (5/16"))
C	Nail (8D)
C	Screw (up to M4 (5/32"))